1. Field of the Invention
This invention relates to a process for making a graphite film having good flexibility and elasticity and adapted for use as heating elements, structural materials, gaskets, and heat-resistant sealing materials.
2. Description of the Prior Art
Since graphite has remarkably high resistances to heat and chemicals and high electric conductivity, it is very important as an industrial material. In fact, graphite has wide utility in the fields of electrodes, heating elements, structural materials, gaskets, heat-resistant sealing materials and the like. Natural graphite may be used for the above purpose but natural graphite of good quality is obtained only in very limited amounts of production. Natural graphite is usually available in the form of powders or blocks which are difficult to handle. Accordingly, artificial graphite has been produced instead. Especially, film-like graphite does not occur naturally and has to be artificially made.
A typical process of producing artificial graphite is a so-called expansion technique. In this technique, natural graphite is immersed in a mixture of concentrated sulfuric acid and concentrated hydrochloric acid and subsequently heated to expand graphite layers, thereby obtaining artificial graphite. The thus obtained graphite is then washed to remove the acid mixture and pressed at high pressure to obtain a graphite film.
However, the graphite film obtained in this manner is nothing like natural single crystal graphite with respect to various characteristics. For instance, the electric conductivity is usually at about 1.5.times.10.sup.3 S/cm whereas the single crystal graphite has an electric conductivity of 2.0.times.10.sup.4 S/cm. The film strength is poor because of the formation from powder. In addition, large amounts of the acids are necessary for the preparation of the artificial graphite with an attendant problem on the generation of SO.sub.x and NO.sub.x gases. The use of the graphite film has the problem that since the acids used for the preparation of graphite cannot be completely removed, corrosion of metals which are used in contact with the film occurs owing to the leaching-out of the residual acid. When the known graphite film is applied as a gasket, the above problems become more serious. In fact, the graphite gasket which has been obtained in a manner as stated above has never been employed in high temperature and high pressure systems or vacuum systems which have to be completely free of sulfur or acids. Moreover, since a starting material is graphite powder, a difficulty is involved in obtaining a thin film whose thickness is less than 0.1 mm with a limitation placed on film strength. Accordingly, there is a demand for the development of a process for making graphite with high quality without resorting to the known expansion technique.
To solve the above problems, we developed and proposed a process for obtaining a graphite film directly from a film of a specific type of polymer by thermally treating the polymer film for graphitization. This thermal treating process is far simpler and easier than the known technique, from which not only there is obtained graphite whose physical properties are nearly as good as those of single crystal graphite, but also any problem of the residual acid is not involved. However, this process is disadvantageous over the expansion technique in that only a relatively thin graphite film is obtained. More particularly, if the starting film has a thickness of not larger than 25 .mu.m, a graphite film having a thickness within 15 .mu.m can be obtained according to the thermal treatment procedure. Over 25 .mu.m, it is usual that because of the generation of gases from the inside of the starting film during the thermal treatment, the resultant film becomes ragged on the surfaces thereof. Thus, film-like graphite with good quality cannot be obtained. The mere heating of polymer film makes it difficult to obtain a graphite film whose thickness is over 15 .mu.m.